Dr. Ledbetter's laboratory is interested in both the mechanisms causing chromosomal abnormalities such as deletions, marker chromosomes, and uniparental disomy, and the consequences of these events in terms of gene dosage imbalance and genomic imprinting effects on human development. The laboratory uses a broad array of conventional cytogenetic, molecular cytogenetic, and molecular biology technologies to investigate several human dosage and/or imprinting disorders as model systems. Lissencephaly, a neuronal migration defect leading to profound mental retardation, is used as a model for a haploinsufficiency disorder in man. Deletions within chromosome band 17p13.3 are detected by fluorescence in situ hybridization (FISH) in 40% of patients with isolated lissencephaly and >95% of patients with the Miller-Dieker syndrome. A previous candidate gene identified in this region now appears to map proximal to the smallest deletion interval for lissencephaly. Current studies focus on a balanced translocation breakpoint in a lissencephaly patient, which has been mapped to a <100 kb interval. Two new transcripts have been identified in this region and are being characterized as potential candidates. Prader-Willi (PWS) and Angelman syndromes (AS) are distinct mental retardation disorders caused by deficiency of a paternally expressed gene(s) or maternally expressed gene(s), respectively. We have cloned the critical region on chromosome 15 in a 3.5 Mb YAC contig, and refined the smallest interval for PWS to about 200 kb and for AS to about 1 Mb. Studies in our lab focus on narrowing the critical region for each disorder, identification of candidate genes imprinted in the appropriate direction, development of improved diagnostic methods and probes for these diseases, and determination of the mechanism of chromosome deletion and other rearrangements in this region. Significant advances in the past year include development of an improved DNA methylation assay for detection of all forms of PWS and 3 of the 4 forms of AS. Additionally, studies of trisomy 15 mosaicism have determined that the majority of these are meiotic in origin with a high risk of uniparental disomy (UPD) resulting from secondary chromosome loss. This finding will have major implications for management and diagnostic testing of trisomy mosaicisms detected prenatally.